1. Field of the Invention
This invention relates to a projection type liquid crystal display apparatus and, more particularly, it relates to a projection type liquid crystal display apparatus comprising a plurality of liquid crystal light valves.
2. Related Background Art
In recent years, projection type liquid crystal display apparatus capable of displaying high definition bright images have been developed and are popularly used as presentation tools for business applications. Such projection type liquid crystal display apparatus typically comprise three liquid crystal panels of the three primary colors of red (R), green (G) and blue (B) particularly when they have a large number of pixels and adapted to display highly bright images.
FIG. 5 of the accompanying drawings schematically illustrates a typical projection type liquid crystal display apparatus comprising three liquid crystal panels. The apparatus also comprises transmission type liquid crystal light valves. Referring to FIG. 5, cabinet 101 contains a light source 102 for converging rays of light from a light emitting tube containing halide or xenon therein and adapted to emit collimated light and six mirrors for separating the light from the light source into rays of light of R, G and B and combining them appropriately (including a B reflector mirror 103 for selectively reflecting B light, a first R reflector mirror 104 for selectively reflecting R light, a first reflector mirror 105, a second reflector mirror 106, a second R reflector mirror 107 for selectively reflecting R light and a G reflector mirror for 108 selectively reflecting G light) which illuminate an R light valve 109, a G light valve 110 and a B light valve 111 with rays of light of R, G and B respectively. The rays of light modulated by the respective light valves are combined to produce an image, which is then enlarged by a lens group 112 and projected onto an external screen 113. In a projection type liquid crystal display apparatus, the light source 102 emits a large quantity of light emitted per unit time and highly concentrated energy of light that amounts to a million luxes will be made to pass through each of the light valves to raise the temperature of the latter, which will then have to be cooled typically by means of an air cooling fan 114. On the other hand, each of the light valves 115 may have to be provided with a heater 115 to raise its temperature in order to improve its responsivity when it is operating at low temperature. Additionally, since the quantities of light entering the respective light valves of R, G and B differ from each other, the temperature rise may differ among the light valves. In other words, the graph of FIG. 6 showing the relationship between the liquid crystal transmittance and the applied voltage of any of the liquid crystal light valves will generally shift to the lower voltage side as the temperature of the light valve rises. For example, the relationship may shift from that of the solid line in FIG. 6 to that of the broken line. Furthermore, since the temperature generally differs among the light valves of the three different colors, the transmittance will also differ among the light valves as T1 and T2 in FIG. 6 for a same applied voltage V. This will result in color distortions of the displayed image. Such color distortions can become remarkable particularly when the quantity of light emitted from the light source gradually rises over several minutes after turning on the power switch or when the ambient temperature surrounding the apparatus changes dramatically. With a known technique for dealing with this problem, each of the light valves is provided with a heater 115 or with a temperature sensor 116 and a heater and the temperature of the light valve is controlled by a controller 117. It is also possible to control the temperature of the light valve by means of a Peltier device (not shown). However, such known temperature control techniques are costly and provide an obstacle to the attempt of reducing the cost of manufacturing liquid crystal display apparatus.
As discussed above, with known projection type liquid crystal display apparatus comprising a plurality of liquid crystal light valves, the latter show temperature difference depending on their colors to give rise to color distortions.
The problem of color distortions becomes remarkable particularly when the quantity of light emitted from the light source gradually rises after turning on the power switch or when the ambient temperature surrounding the apparatus changes dramatically.
The use of a temperature controller, a temperature sensor, a heater and/or a Peltier device for controlling the temperature of each of the light valves, however, results in a large and costly display apparatus.